This invention relates to a blow-molded foam having a high expansion ratio and a large wall thickness and also to a process for producing the same.
It has been hitherto performed to obtain hollow blow-molded foams having a foamed layer by use of blow molding. As a process of obtaining blow-molded foams, there have been proposed a number of processes, of which there has been frequently used a process, which comprises melt kneading a foaming agent and a base resin by means of an extruder, placing a cylindrical foamed parison, which has been formed by extrusion of the melt from a die, in a mold, and subjecting the parison to blow molding wherein a pressurized gas is blown into the parison.
Such blow-molded foams have utility in the fields which require heat insulation properties, soundproof properties, flexibility and the like, particularly, in the fields of ducts, automotive parts, containers and the like. In these applications, there have been favorably employed foams whose expansion ratio is at 3xcx9c30. For instance, in Japanese Patent Publication No. Hei 3-59819, there is disclosed a blow-molded foam whose foamed layer is made of polystyrene or polyethylene as a base resin and has an expansion ratio in the range of 4xcx9c12. Taking the heat resistance, rigidity and the like of the molded foam into account, it is preferred to use a polypropylene resin as a base resin.
However, when an attempt has been made to obtain a blow-molded foam while using a polypropylene resin as a base resin, there could not have been obtained, according to prior art techniques, blow-molded foams having a favorable expansion ratio of 3xcx9c30 and a thickness of not smaller than 0.5 mm. For instance, in Japanese Laid- open Patent Application No. Hei 8-72127 wherein an attempt is made to obtain products of a high expansion ratio, the expansion ratio of the resultant blow-molded foam is, at most, as low as 3, with a difficulty being involved in obtaining a foam having a higher expansion ratio. Moreover, the thickness of the, foamed layer of the blow-molded foam is no more than approximately 0.3 mm, and it has been difficult to obtain a product having a larger thickness.
The reason why it is not possible to obtain a blow-molded foam having an expansion ratio of 3 or over and a thickness of 0.5 mm or over in case where a polypropylene resin is used as a base resin is considered as follows.
When a molten mixture of a base resin and a foaming agent is extruded from the die of an extruder toward the low pressure zone, the foaming agent mixed with the base resin is expanded, thereby causing foaming, whereupon when the temperature of the base resin is high, the viscosity and melt tension so lower as not to keep the foaming agent to be held in the base resin. This entails dissipation of the foaming agent from the base resin matrix at the time of extrusion foaming, thereby forming an open-cell foamed parison, or in the worst case, making the foaming impossible. In contrast,e if the temperature of the base resin is lowered in order to keep the viscosity and melt tension of the base resin at high levels, the crystallization of the base resin proceeds, disabling the satisfactory and uniform expansion.
Accordingly, the extrusion foaming has to be performed at a temperature at which satisfactory and uniform expansion proceeds and the base resin exhibits viscoelasticity sufficient to hold the foaming agent therein. The temperature range, within which there is obtained viscoelasticity suitably adapted for foaming, differs depending on the type of resin and the amount of foaming agent, and is generally called proper foaming temperature range. The polypropylenie resin having a high degree of crystallization has viscoelasticity which greatly varies depending on a slight change of temperature when compared with other types of resins such as polystyrene, polyethylene and the like.
In this way, the proper foaming temperature range of polypropylene resin is very narrow, and when it is attempted to obtain a blow-molded foam at a relatively low expansion ratio, the ratio of a base resin to a foaming agent being added becomes so large that even though the viscoelasticity of the resin is low, a foaming agent can be satisfactorily held in the resin, thereby relatively easily keeping the extrusion temperature within a proper foaming temperature range. On the contrary, however, where the amount of a foaming agent is increased so as to obtain a blow-molded foam having a high degree of foaming, viscoelasticity sufficient to withstand foaming cannot be maintained. Especially, with blow molding, the parison extruded from a die should be in a softened condition until it is shaped in a mold. Eventually, to keep the softened state presents the drawdown problem caused by the weight of the foamed parison. In view of the relation with the melt viscosity capable of preventing the drawdown, limitation is placed on the extrusion temperature for the foamed parison, which makes it more difficult to set the extrusion temperature within a proper foaming temperature range.
We made further intensive studies based on the above knowledge, and as a result, arrived at the completion of the invention.
The present invention has for,its object the provision of a blow-molded foam of the type which is obtained by use of a polypropylene resin as a base resin, the product having a high expansion ratio which would have been difficult to attain in prior art techniques and has a large wall thickness.
The invention has as another object the provision of a process for producing a blow-molded foam which is able to produce a blow-molded foam having a high expansion ratio and a large wall thickness in a simple way.
The invention provides a blow-molded foam which is obtained by blow molding a parison in a desired form and has a foamed layer made of a base resin mainly composed of a polypropylene resin and in which a resin forming the foamed layer has, at 230xc2x0 C., a melt tension (MT) of 5 gf or over and a melt flow rate (MFR) of 1 g/10 minutes or over, and the foamed layer has an average cell diameter of 0.05xcx9c2.2 mm, an average thickness of 0.5 mm or over, and an average density of 0.30 g/cm3 or below.
The invention encompasses an arrangement wherein the foamed layer includes, as an inner layer and/or an outer layer, other layer or layers. In this case, other layer may be constituted of a resin, a woven fabric, a non-woven fabric, a metallic foil, a rubber or the like.
Where other layer is formed of a resin, the resin should preferably be one which comprises a polypropylene resin as its main component and has a density exceeding 0.3 g/cm3. The resin whose density exceeds 0.3 g/cm3 may be either a foamed resin or a non-foamed resin.
The resin having a density exceeding 0.3 g/cm3 should preferably have such properties of a melt tension (MT) of 5 gf or over and a melt flow rate (MFR) of 1 g/10 minutes or over at 230xc2x0 C. Moreover, the resin layer made of the above resin should preferably have an average thickness of 1xcx9c7 mm.
For the manufacture of the blow-molded foam of the invention, a foaming agent is added to a base resin which is mainly made of a polypropylene resin and has, at 230xc2x0 C., a melt tension (MT) of 10 gf or over and a melt flow rate (MFR) of 0.5 g/10 minutes or over and melt-kneaded under high temperature and high pressure conditions by means of an extruder to provide a foamable melt. This melt is extruded from a die into a low-pressure zone to form a, parison having a foamed layer. Next, the parison in a softened condition is placed in a mold, and a pressurized gas is blown into the parison to obtain a blow-molded foam having the foamed layer.
Although a resin comprising a polypropylene resin as a main component is used as a base resin, the blow-molded foam of the invention has a foamed layer having an expansion ratio of 3 or over, and a thickness of 0.5 mm or over and can fully satisfy physical properties required for the blow-molded foam, such as a heat IBM resistance, heat insulation properties, sound proof properties, flexibility, rigidity and the like. Thus, the foam product can be favorably used in various applications such as of ducts, automotive parts, containers, electric appliance members and the like.
According to the process for producing the blow-molded foam of the invention, a novel,blow-molded foam, which has a foamed layer formed of a base resin comprising a polypropylene resin as a main component and having an expansion ratio of 3 or over and a thickness of 0.5 mm or over and which is excellent in surface smoothness, can be readily obtained.